I looked recently at the walkabout but went with the explorer. The big differences I could see were the arrangement of the pockets, the way the outside lower flap covered the lover pockets and the side pockets.
On the walkabout the pockets for a mono-pod with a retaining strap are very visible. On the explorer the pocket hides away in its own flap and is more suited for water bottles (one on each side).
Construction seems about the same for each, and while expensive remember some of the money you spend on it does go to support National Geographic programs and support wildlife conservation.